Nuclear magnetic resonance (NMR) measuring apparatuses and electron spin resonance (ESR) measuring apparatuses are conventionally known as representative magnetic resonance measuring apparatuses. Magnetic resonance imaging (MRI) apparatuses are known as being similar to the NMR measuring apparatuses. Hereinafter, the NMR measuring apparatus will be described in detail below.
The NMR is a phenomenon caused by atomic nuclei placed in a static magnetic field that interact with electromagnetic waves of specific frequencies. The NMR measuring apparatus is an apparatus capable of utilizing such a phenomenon to measure a sample at an atomic level. The NMR measuring apparatus can be practically used in analyses of organic compounds (e.g., chemicals and pesticides), high polymer materials (e.g., vinyl and polyethylene), and biological materials (e.g., nucleic acids and proteins). For example, the NMR measuring apparatus enables a user to examine the molecular structure of a sample.
The NMR apparatus includes an NMR probe (i.e., NMR signal detection probe) placed together with a sample in a superconducting magnet that generates a static magnetic field. The NMR probe includes a detection coil for transmission and reception. The detection coil has a function of applying a variable magnetic field to the sample in a transmission state and a function of receiving an NMR signal from the sample in a reception state. The resonance frequency is variable depending on an observation target nuclide. Therefore, in the measurement of the sample, a high-frequency signal having a particular frequency adapted to the observation target nuclide is given to the coil.
The uniformity of an external magnetic field in a sample space plays an important role in improving the detection accuracy of the NMR signal. If the external magnetic field is not uniform in the sample space, the NMR signal may be erroneously detected. The cause of generating such a non-uniform external magnetic field is, for example, magnetization of a member that approaches the sample space. In particular, the detection coil for detecting the NMR signal may generate a disturbed magnetic field in the sample space if the magnetization of the coil is finite. In general, an NMR apparatus having higher resolution includes a shimming apparatus that can correct the magnetic field distribution in the sample space. However, the practical order of the correction is limited to a lower order. If a detection coil has a complicated shape, it is generally difficult to correct a non-uniform magnetic field caused by the magnetization of the coil. Therefore, in a detection coil that approaches the sample space and has a complicated shape, the magnetization of the material must decrease down to zero as much as possible.
Further, it is conventionally known that Faraday's law of electromagnetic induction is employable in a method for detecting the NMR signal. The noise dominant in this method is Johnson noise. The Johnson noise is known to be proportional to the square root of the coil temperature or the square root of the electric resistance of the coil.
As discussed in the reference “High Temperature Superconducting Radio Frequency Coils for NMR Spectroscopy and Magnetic Resonance Imaging,” Steven M. Anlage, “Microwave Superconductivity,” ed. by H. Weinstock and M. Nisenoff, (Kluwer, Amsterdam, 2001), pp. 337-352, a detection coil made of a superconductor is conventionally known. When a superconductor is cooled, the electric resistance of the superconductor becomes substantially zero. Therefore, the above-mentioned noise can be reduced and the detection sensitivity of the NMR signal can be improved.
Selecting a superconductor as a detection coil material is useful in that the thermal noise signal can be reduced. However, the superconductor possesses strong magnetic shield characteristics induced by superconductive phenomenon. The above-mentioned property possibly causes a disturbance in the uniformity of a magnetic field in the sample space. As a result, the sample filling rate (i.e., the ratio of the volume of a measurement target sample to the volume of the detection coil) decreases and the S/N ratio of the NMR signal decreases.
The sample filling rate is mentioned in the reference “High Temperature Superconducting Radio Frequency Coils for NMR Spectroscopy and Magnetic Resonance Imaging,” Steven M. Anlage, “Microwave Superconductivity,” ed. by H. Weinstock and M. Nisenoff, (Kluwer, Amsterdam, 2001), pp. 337-352. When the sample filling rate of a metallic detection coil in a room-temperature environment is 1, the sample filling rate of a superconductive detection coil in a low-temperature environment is approximately 0.2. In general, the detection sensitivity is proportional to the square root of the sample filling rate. Therefore, if the employed detection coil is made of a superconductor, a relative reduction in the detection sensitivity is approximately 0.45 times. One of the reasons why the sample filling rate of a superconductive detection coil is kept low is that the superconductive detection coil is forcibly isolated from the sample to avoid the non-uniformity of the magnetic field caused by the superconductive detection coil. The sample filling rate decreases with increasing distance between the detection coil and the sample. As a result, the detection sensitivity decreases correspondingly.
As discussed in the reference “Design, construction, and validation of a 1-mm triple-resonance high-temperature-superconducting probe for NMR” William W. Brey et. al., Journal of Magnetic Resonance 179 (2006) 290-293 and U.S. Pat. No. 5,565,778, it is conventionally known that the superconductive detection coil can be configured to include a slit to suppress the generation of shield current that may cause magnetic shield generation. Providing such a slit is effective in suppressing the generation of large shield current. However, smaller residual shield current may flow through a plurality of coil portions separated by the slit. The entire amount of the shield current rises up to a level corresponding to a sum of the separated coil portions. Further, the above-mentioned slit is not effective in preventing a magnetization component parallel to the magnetic field from generating a non-uniform magnetic field.
The present disclosure intends to provide a magnetic resonance signal detection module including a detection coil made of a superconductor, which can improve the detection sensitivity while suppressing the reduction of the sample filling rate of the detection coil as much as possible.